1. Field of the Invention
The present invention relates to transmit power control of a downlink dedicated physical channel (DPCH) in a mobile communication system using wideband code division multiple access (W-CDMA).
2. Description of the Related Art
In a W-CDMA mobile communication system, a technology called closed-loop transmit power control using both inner loop power control and outer loop power control is used (e.g., see 3GPP TS 25.214 V4.1.0 (2001-06)).
In this closed-loop transmit power control, a receiving end measures a quality of a communication channel, and transmits a transmit power control (TPC) signal based on the measurement result by using a dedicated physical control channel (DPCCH) on a line loop-back to a transmitting end. The transmitting end receives the TPC signal and controls the transmit power value of the communication channel on transmission based on the UP/DOWN command appended to the TPC signal. Therefore, it is controlled so that the downlink communication channel in the receiving end satisfies required quality.
In the inner loop power control of one of them, as shown in FIG. 4, first, the receiving end (mobile terminal 1 in FIG. 4) measures a signal to interference ratio (SIR). Next, the receiving end compares the measured SIR and a target SIR. If the measured SIR is smaller than the target SIR, the receiving end transmits the TPC signal with an “UP” command to the transmitting end (radio base station 2 in FIG. 4), meanwhile, if the measured SIR is larger than the target SIR, the receiving end transmits the TPC signal with a “DOWN” command to the transmitting end. The transmitting end which has received the TPC signal changes the transmit power of the downlink communication channel by +/−1 dB based on the result of decoding the TPC signal. The receiving end performs such power control in a slot cycle. In addition, the slot cycle is one-fifteenth of one radio frame cycle which is 10 ms, that is, the slot cycle is 0.667 ms.
In another outer loop power control, the receiving end changes the target SIR value in the above-mentioned inner loop power control so that a communication quality value such as a bit error rate (BER) or a block error rate (BLER) satisfies a target quality value. For example, the receiving end measures a BLER which is one of communication quality values, and compares the measured BLER and a target BLER. Based on the comparison result, the receiving end increases the target SIR if the measured BLER is smaller than the target BLER, or decreases the target SIR if the measured BLER is larger than the target BLER. In addition, this communication quality value may be calculated by measuring continuously during a certain period (several hundred milliseconds to several seconds).
In the above-mentioned conventional closed-loop transmit power control, that is, transmit power control using inner loop power control and outer loop power control, particularly in the case of a communication environment in which the interference signal code power (ISCP) is low, the mobile terminal 1 continuously transmits the TPC signal with a “DOWN” command because the measured SIR is very larger than the target SIR.
As the radio base station 2 which has received the TPC signal continuously decreases the transmit power of downlink DPCH, the received power at the mobile terminal also continues to decrease. In the mobile terminal, the communication quality value is measured at the next timing, and the target SIR is increased at the time when it has been revealed that the measurement result does not satisfy the target quality value. However, an error occurs in order that the increment of the target SIR can not fulfill a proper value,
Thus, in the transmit power control using inner loop power control and outer loop power control, the transmit power control suitable for communication environment cannot be performed because the inner loop power control differs in an updating cycle of power control from the outer loop power control greatly.